Prince or Princess: Royal succession changes mean baby will be monarch
Whether the Duke and Duchess of Cornwall have a son or a daughter the baby will be the third in line to the throne after proposed changes to the royal succession rules.
In October 2011, David Cameron announced that the 16 Commonwealth countries where the Queen is head of state had agreed to give female royals the same rights of succession as their brothers.
Under the ancient rules of male primogeniture, first born royal daughters in direct line to the throne were leapfrogged by their younger male siblings.
Mr Cameron had previously warned it would take time due to on-going negotiations with the Commonwealth countries where the Queen is also head of state, but at the Commonwealth Heads of Government Meeting in October 2011 he confirmed the changes would go ahead.
It is understood that proposed legislation will refer to the descendants of the current Prince of Wales, meaning that William's first child would follow him in the line of succession, whether a girl or a boy.
However, the current generation of royals will not be affected. It will not be retrospective - meaning the Princess Royal will not jump ahead of her younger brothers the Duke of York and the Earl of Wessex.